On-going studies of the differentiation of chick embryo intestine in organ culture will be continue. Attention will be principally focused on the development of the microvillus membrane, with specific reference to alkaline phosphatase, maltase, and sucrase, which are constituents of the membrane. Thyroxine, which strongly accelerates the differentiation of intestinal epithelium in culture, will be used, but further efforts will be made to determine the time and circumstances under which glucocorticoids influence the differentiative process. The possible intervention of cAMP (and/or cGMP) will be examined. Short-term exposure to cycloheximide having indicated differences in the post-translational processing of alkaline phosphatase and maltase, we plan to investigate the influence of prior treatment with cycloheximide on glycosylation and transport of membrane proteins, as revealed by uptake and distribution of labelled sugars.